Over the last several decades, different types of insoluble substances have been found to be highly active and selective catalysts for transforming olefins such as propylene to epoxides such as propylene oxide using active oxygen species. One class of such catalysts includes the titanium silicalites such as TS-1 and other zeolites having titanium atoms in their framework structures, which work well where the oxidant is hydrogen peroxide and the olefin is relatively small. See, for example, U.S. Pat. No. 4,833,260. When the active oxygen species is an organic hydroperoxide such as ethyl benzene hydroperoxide, the use of porous amorphous catalysts such as those commonly referred to as "titania-on-silica" is preferred. Olefin epoxidation using such catalysts is described, for example, in U.S. Pat. No. 4,367,342.
Although heterogeneous epoxidation catalysts typically exhibit high activity and selectivity when freshly prepared, gradual deactivation takes place simultaneous with epoxidation. This problem is particularly acute in a large scale continuous commercial operation where, for economic reasons, an epoxidation process must be capable of being operated over an extended period of time while maintaining high yields of epoxide. Although regeneration methods for such catalysts are known, it would be highly advantageous to develop procedures whereby the interval between regenerations is extended for as long as possible. Regeneration requires that epoxidation be interrupted for some period of time sufficient to effect catalyst reactivation, thereby reducing the effective annual capacity of a commercial plant. The deactivated catalyst could alternatively be replaced with fresh catalyst, but the same practical disadvantages will result as with regeneration. Additionally, catalysts of this type tend to be relatively costly and it would be desirable to minimize the quantity of fresh catalyst which is needed to supply the plant.